High Modulus Steels

Steels are a part and parcel of the modern society and find application on a wide spectrum. One of the major areas where steels are extensively used is the automotive sector because of their excellent combination of strength, formability, affordability and recyclability. On the one hand, the automotive industry accounts for a major portion of the economy in the advanced world (e.g. 3–3·5% of gross domestic product in USA),¹ but at the same time, automobiles are also the second largest source of greenhouse gas emissions in the world, next only to the thermal power plants. This has attracted stringent environmental regulations urging the automotive manufacturers to reduce emissions.² Weight saving in the automobiles would be a natural direction for effecting fuel efficiency as well as bringing down emission. In a typical passenger car the body-in-white (BIW) accounts for around 36% and other body parts (closures, bumper etc.) another about 14% of the total curb weight. Consequently, there is an urgent pressure on the steel producers to innovate and introduce new grades of steels that can reduce the weight of the BIW and other body parts.

Research and development on automotive steels have traditionally focused on increasing the strength while maintaining adequate formability.³ The development and use of high and ultra high strength steels are based on the logic that the higher levels of strength would enable the automotive designer to go for a lower sheet thickness and hence result in weight saving.⁴ However, steel sheets cannot be down-gauged to any extent without compromising the stiffness.⁴ Excessive reduction of sheet thickness may lead to an unacceptably lower level of structural rigidity for several desirable attributes (e.g. low noise, vibration and harshness (NVH), low bending deflection etc.), notwithstanding the fact that the strength of the steel is high. Therefore, stiffness rather than strength would be the critical criterion for materials selection in auto body components. Moreover, the stiffness of a structure is of principal importance in selection of materials for various other engineering sectors such as aerospace, transport, building and construction, heavy equipment and machinery etc.⁵

The stiffness of a sheet is influenced by both the density and the elastic or Young’s modulus.⁶ A lower density would allow use of a thicker gauge that increases the stiffness, and a higher elastic modulus on the other hand directly increases the stiffness. Both these approaches of lowering the density and enhancing the elastic modulus of steels are relatively new fields in the horizon and are, therefore, less focused areas in the literature.

In the above back-drop, the present collection of invited papers on ‘high modulus steels’ is timely and useful both for the academic as well as industrial research communities. An attempt has been made here to put together reviews^7–12 and original contributions^13,14 from colleagues around the globe, known to be active in the area of interest for this compilation. A brief introduction to the individual papers is given below.

The elastic modulus, being an intrinsic property of a material, is very difficult to alter. As reviewed thoroughly by Bonnet et al.,⁷ Akhtar⁸ and partially by Münstermann et al.,⁹ various ceramic particles such as oxides, carbides, nitrides and borides with significantly high elastic modulus can be reinforced into steel in order to develop a steel-matrix composite (SMC) that may constitute the most effective solution. The elastic modulus of steels can be greatly improved through SMCs by adding considerable amounts of second phase particles. The constraint here is the fact that fabrication of SMCs could entail expensive synthesis techniques and also put limitations on quantity production.⁸ However, Bonnet and co-workers⁷ indicated that with an appropriate selection of ceramic particles, eutectic solidification of the alloy can be achieved in a conventional industrial process, involving a continuous caster and a hot strip mill, that can guarantee high productivity and low cost. The microstructure development in the SMCs is of particular importance for controlling the mechanical properties.⁸ It is believed that absence of reinforcement segregation and a uniform distribution of ceramic particles in steel matrix ensure an isotropic microstructure, uniform distribution of thermal stresses and superior mechanical properties.⁸ A strong and defect-free interface between the steel matrix and the reinforcing particles leads to effective load transfer and, therefore, is important in achieving good strengthening.

Rana and Liu¹³ reported the effects of TiB₂ particles on the elastic modulus and mechanical properties of ferritic steels produced by conventional melting and casting route. About 13 vol.% TiB₂ particles in a ferritic matrix caused a 15% increase of elastic modulus in transverse direction after cold rolling and annealing, with a concomitant 5·7% decrease in density of the material. The study also highlighted the influence of thermo-mechanical processing steps on the deterioration of the structure at the particle-matrix interface, leading to a decrease in elastic modulus that was achieved in the as-cast condition. Dalai and co-workers¹⁴ fabricated a TiC-reinforced high manganese austenitic steel matrix-composite via traditional melting and casting route, and analysed the microstructure-mechanical property relationship with reference to the base steel. A 16% increase in elastic modulus of the austenitic steel has been observed with approximately 10 vol.% TiC. An additional 16% increase in elastic modulus was achieved through a secondary processing employing hot rolling of as-solutionised samples probably due to reduction in porosity, albeit at a low energy absorption capability.

Münstermann et al.⁹ reviewed the metallurgical factors influencing the elastic modulus of steels. Some of the substitutional alloying elements in ferrite can influence the elastic modulus at large concentrations by altering the interatomic distance and modifying the distribution of electrons. In this context, Rana and Liu¹³ observed a drop in elastic modulus in low-density ferritic steels due to addition of Al and an increase of the elastic modulus by Mn in ferrite-based duplex phase low-density steels. They have also focused on alloying strategies to vary the phase fractions of ferrite and austenite that can affect the elastic modulus of the alloy. A higher temperature and presence of plastic deformation usually reduce the elastic modulus in steels.⁹ Low temperature heat treatment involving strain ageing (paint baking) in bake hardening steels can cause recovery of the elastic modulus, lowered due to prior plastic deformation.⁹ Texture can have significant effect on elastic modulus of steels. Pramanik and co-workers¹⁰ prepared a comprehensive review on the microstructural aspects affecting the elastic modulus of steels with an emphasis on texture. Considering different models, based on averaging procedures of texture, elastic modulus of polycrystalline steels can be computed. The elastic modulus for a single crystal of iron exhibits a maximum value along <111>, and a minimum along <100> direction. In polycrystalline steels, elastic modulus varies with the angle to the rolling direction for different texture components. Thus, P-containing and Al-killed low carbon steels show the maximum elastic modulus at angles of 90° and 45° to the rolling direction, respectively, due to development of different textures. The review by Sandström and Korzhavyi¹¹ focused on the prediction of elastic constant as a function of composition of austenitic stainless steels, employing ab-initio quantum mechanical methods. The paper underlined the importance of computational techniques to perform materials optimisation by choosing stiffness as the controlling property along with an aim for minimisation of cost, weight and environmental impact.

Accurate measurement of elastic modulus is of paramount importance to the development of high modulus steels, as a high scatter in results can be expected with varying measurement techniques.⁹ Lord and Morrel¹² compared the merits of available static and dynamic methods of elastic modulus measurement for steels and metal-matrix composites. A static method such as tensile test yields an ‘engineering value’ of elastic modulus; however to achieve a good accuracy in data with this method, use of a dedicated test with high accuracy strain measurement, loading below the elastic limit, careful machine and test piece alignment and robust data analysis procedures are necessary. On the other hand, the determination of elastic modulus is quick, non-destructive, and is of high inherent accuracy when dynamic techniques are employed.

Finally, it may not be even necessary to state that it is not possible to cover all the aspects of high modulus steels in one collection of papers. For instance, while some of the fundamental aspects have been covered here, issues relating to thermo-mechanical processing and formability of these steels still need to be addressed at a greater length. In any case, it is believed that the present collection of papers would serve as an important step in looking into the generation of ‘High Modulus Steels’ as a new group. I am grateful to all the authors for their contributions through the excellent articles that have made this publication possible.

formerly at: Tata Steel Europe, 1970CA IJmuiden, The Netherlands

currently with:

Advanced Steel Processing and Products Research Center

The George S. Ansell Department of Metallurgical and Materials Engineering

Colorado School of Mines

Golden, Colorado 80401, USA

E-mail: [email protected]; [email protected]

Telephone: [phone]